Method for reducing the risk of poor reception in a mobile telephony system

Abstract

A method of reducing in a digital time multiplex mobile telephony system the risk of poor reception as a result of the occurrence of points of low signal strength, so-called zero points. Transmission from fixed stations is performed with a polarization direction which is changed stepwise between different parts of the transmitted information. This causes the fading patterns around receiving stations to change in time, and also causes any zero points to be moved in the geography, therewith making it improbable that more than a small part of an information frame will be lost. In the case of a GSM-type system, the polarization direction is changed for each new TDMA-frame.

I claim: 
     
       1. A method for reducing in a digital time multiplex mobile telephony system the risk of poor reception when transmitting from a fixed station to a mobile station, as a result of reflection of transmitted signals by different objects, wherein each information frame to be transmitted is divided into a number of information parts determined by the number of interleaving levels, comprising the steps of:   transmitting from the fixed station with a polarization direction; and   changing said polarization direction stepwise in a manner such that all information parts from one and the same information frame are transmitted in mutually different polarization direction;   wherein the number of different polarization directions coincides with the number of interleaving levels.   
     
     
       2. A method according to claim 1, wherein during said transmitting step, the aforesaid information parts from one and the same information frame are included in a respective TDMA-frame, and during said changing step the polarization direction is changed after each TDMA-frame. 
     
     
       3. A method according to claim 1, wherein the polarization direction is changed after each transmitted information part. 
     
     
       4. A method according to claim 1, further comprising the steps of: supplying at least two antennas which are oriented with mutually different polarization directions each with respective signals which have mutually the same frequency and mutually the same information content; and   modulating the amplitudes of said signals in a manner such that the polarization direction of a resultant output signal from the antennas is changed as in said changing step.